1. Field of the Invention
The present invention relates to a gas sensor that is attached to an intake system (intake pipe or the like) or an exhaust system (exhaust pipe or the like) of an internal combustion engine, such as a diesel engine or a gasoline engine, and that is used to detect the concentration of a specific gas in a gas to be measured (detected), such as an intake gas or an exhaust gas.
2. Description of the Related Art
Gas sensors of this type include a detection element (also referred to as an “element”) having a strip-like shape or a bar-like shape (shaft-like shape). A front end portion of the element serves as a detection portion, and a pair of electrodes (detection electrodes) are disposed on the front end portion. Examples of such gas sensors include a gas sensor (also referred to as a “sensor”) that includes a metal shell, which has a tubular shape so that the gas sensor can be attached to a predetermined portion (attachment hole) of an object, such as an intake pipe or an exhaust pipe of an engine. The element of the sensor extends through the inside of the metal shell in the front-rear direction and fixed in position so that the detection portion protrudes from the front end of the metal shell. The sensor is airtightly attached to an attachment hole of an object so that the front end of the element is exposed to a gas to be detected and the element generates an electric signal in accordance with a gas concentration difference. The electric signal is output through electrode terminals, which are disposed on a portion of the element adjacent to a rear end of the element, to an engine controller unit (ECU), which is disposed outside the sensor, and is used to control the air-fuel ratio and the like.
Examples of means for attaching the gas sensor to the attachment hole and means for airtightly attaching the gas sensor include the following. One example is a method of screwing a metal shell, which has a threaded outer peripheral surface, into a threaded hole in the object. Another example is a press-fitting method in which an elastic sealing member, such as a ring-shaped rubber packing, is fitted and mounted into a groove formed in the outer peripheral surface of the metal shell, and the metal shell is press-fitted (pressed) into the attachment hole against a force generated by elastic deformation of the elastic sealing member (rubber packing) (Japanese Unexamined Patent Application Publication No. 2012-251852). With the press-fitting method, the rubber packing is compressively deformed between the inner peripheral surface of the attachment hole and the outer peripheral surface of the metal shell, and a gap between the inner and outer peripheral surfaces is airtightly sealed by using an elastic force generated due to the deformation. With this method, the sensor can be attached easily and rapidly by only pressing the sensor into the attachment hole in the axial direction. Therefore, this method is suitable for a case where the sensor is not used under severe conditions, such as a case where the gas pressure is low.
The gas sensor described in Japanese Unexamined Patent Application Publication No. 2012-251852 includes a resin protective cover (resin outer tube or the like) that is integrated with a rear portion of the metal shell by injection molding or the like, so that the structure of the gas sensor can be simplified. The protective cover accommodates and covers the rear end of the element, lead wires that are electrically connected to the element, connectors (metal terminals), and the like. The injection molding is performed, for example, by using a part (rear portion) of the metal shell as an insert (a component to be integrally embedded in a resin). The resin protective cover includes a cylindrical cover portion, which has a cylindrical shape substantially coaxial with the metal shell. The cylindrical cover portion covers the rear end of the detection element, which protrudes rearward from the metal shell, and metal terminals that are electrically connected to the element. An operator holds the cylindrical cover portion with his/her fingers when attaching the sensor to an attachment hole by using the press-fitting method. In the present application, the term “grip” may be used instead of “hold”, and a portion to be gripped may be referred to as a “grip”.
When attaching the sensor described in Japanese Unexamined Patent Application Publication No. 2012-251852 to an attachment hole by using the press-fitting method, it is necessary to press the sensor against an elastic force generated due to deformation of the rubber packing. Accordingly, in this operation, it is not possible to appropriately and smoothly attach the sensor by simply pressing the sensor straightly because the rubber packing (also referred to as the “rubber”) becomes deformed or twisted. Therefore, when press-fitting the gas sensor, usually, the gas sensor is twisted by an appropriate angle in one direction or twisted (alternately) leftward and rightward by, for example, about 90 degrees.
However, fingers of an operator tend to slip over the cylindrical cover portion (grip) of the protective cover of the existing gas sensor described above, because the cover portion is made of a resin and has a cylindrical outer peripheral surface. Moreover, the existing gas sensor has a problem in that, although it is necessary to twist the gas sensor when press-fitting the gas sensor, it is not easy to twist the gas sensor appropriately against deformation of the rubber. Therefore, it has been pointed out that the operation of attaching the gas sensor by using the press-fitting method, which is expected to be easy, cannot be performed easily and rapidly. However, using a dedicated gripping tool to prevent slipping would increase the number of operation steps.
In addition to such a problem about the attachment operation, the existing gas sensor has the following problem. The cylindrical cover portion (grip) of the protective cover of the gas sensor is made of a resin and has a simply cylindrical outer peripheral surface and the outer peripheral surface is smooth (flat). Therefore, in a molding process, the outer peripheral surface (having a cylindrical shape) is likely to be deformed due to, for example, difference in contraction that occurs when the surface is cooled in the molding process. That is, the outer peripheral surface of the cylindrical cover portion (grip) of the existing gas sensor, which has a simply cylindrical and smooth shape, is likely to have defects, such as recesses and bulges, after being molded. Thus, the existing gas sensor, whose surface is smooth, has a problem in that decrease in the quality of appearance is likely to occur, and a molding defect, such as an appearance defect and low dimensional accuracy, may occur.